Billy's dark secret
Billy has noticed that there are foxes near FireClan territory and are stealing all the prey. Billy can't tell Blake. The reason is, the foxes made a deal with him: If you lets us take some of your prey every now-and-then, we won't come and raid your camp, deal? Billy had to say yes, because he was attacked by these foxes as a kit, and he never told anyone about it, so nothing could be done. Billy is afraid of these foxes, and can't do anything.
A death that cannot be told
Brendon witnesses a horrible death, that he cannot tell anyone. A poor kittypet that was Brendon's secret friend named Jake, got attacked by a dog. If he ad told his father, Blake would know that Brendon had been playing with this kittypet. Poor Brendon cannot tell anyone about his best friends death.